OceanStor S2200T storage has 4 disks in total. The customer create a raid 5 group using 3 disks. He got an error when he want to set the fourth disk as hot spare disk.
All of the four disks are coffer disk which are reserved a part of capacity by the system to store system configuration and logs. So,the available capacity is smaller than other non-coffer disks. So coffer disks can't be used as hot spare disk.
adding another disk with the same type(Bom code) as the coffer disk, then set it as hot spare disk.